1. Field of the Invention
The present invention relates to an apparatus using a charged particle beam, such as an electron beam or an ion beam, and in particular to an improvement for a stage position measurement device for detecting the position of a movable stage on which a sample to be exposed is mounted.
2. Related Arts
Since an exposure apparatus using a charged particle beam can perform an exposure at an accuracy on the order of sub-microns, it has drawn attention as a next generation LSI exposure apparatus. In this charged particle beam exposure apparatus, a beam emitted by a beam generation gun is formed into a desired shape when it passes through a predetermined transmission mask, and the shaped beam is deflected to irradiate a desired location on the surface of a sample. Since the range within which the beam is deflected is specifically limited, a stage on which the wafer is mounted is so provided that it can be moved horizontally.
The deflection of a beam is performed by an optical system comprising a beam gun, a deflector, an optical lens, etc. The stage on which the wafer is mounted is located in a chamber to which is attached a column portion in which the optical system is disposed. Therefore, when the stage is moved, it is necessary for the relative positions of the stage and the origin of the optical system to be accurately detected. The distance by which the beam is deflected from the origin of the optical system is calculated based on the relative positions.
A conventional, common stage position measurement device employs, for example, the heterodyne measurement principle. That is, two laser beams having frequencies slightly offset from each other are projected onto a sample stage, and a phase difference between the laser beam reflected by the stage and the reference laser beam that follows an optical path for reference is measured to acquire a displacement distance for the position of the stage. By the application of this principle, it is possible to measure the displacement distance for the position of the stage very accurately.
However, as the external temperature changes, so that the temperature in a clean room wherein the exposure apparatus is installed also changes, a chamber incorporating the stage expands or shrinks at a rate consistent with a thermal expansion coefficient. Accordingly, the distance from the internal wall of the chamber to the origin of the optical system in the column portion, which is normally located in the center of the chamber, is increased or reduced.
Under these circumstances, however, the above described stage position measurement device, which employs a laser interferometer, outputs the displacement distance for the position of the stage without taking into account the fact that the length of an optical path for measurement, along which a beam is projected from the internal wall of the chamber to the stage and is reflected by the stage, is extended or reduced as a consequence of the temperature change. As a result, an incorrect measurement is acquired for the relative distance between the origin of the optical system and the position of the stage.
Such an inaccurate measurement result causes the position irradiated by a beam to be shifted away from a correct location, and causes an exposure failure. The above problem can, in principle, be resolved by providing an expansion or a reduction value corresponding to a temperature change as a compensatory value for the distance a beam is displaced. In reality, however, it is difficult to detect a displacement distance that is attributable to thermal expansion, and since an unwanted compensatory value due to noise could be provided for the displacement distance, the above resolution is not appropriate. Further, a method for limiting temperature changes in a chamber requires the use of a large apparatus, and in actuality, it is difficult to accurately maintain a specific temperature.